Overdrive-Underdrive

motto
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My garage: Harley Davidson 42WLA, Dodge WC63, Dodge WC57, Chevrolet 1-1/2 ton 4x4. Originally an Earth Borer, Pole Setter. Drilling rig gone and truck now mounts a #7 Set Lifting Frame. i.e.: Light Wrecker.
Location: 70km from Melbourne Australia

Overdrive-Underdrive

Post by motto »

Can somebody tell me why it was that during WW2 the US auto industry built over 3/4 of a million 6x6 2-1/2 Ton trucks using an overdrive gearbox and an underdrive transfer case? These features cancel each other out at the expense of an extra gear train in each unit and it baffles me why you would do that.
This question has bugged me for years and I've never heard anyone else even ask it let alone answer it.
The Canadians got it right with their C60X which used a straight through four speed gearbox and a t'fer case that was straight through in high and it was a better truck for it. They may not have the looks of the GMC but always brought much more money in the post-war sales and those fellows wanted value for money.
Even the Chev 1-1/2 Ton 4x4 had both units straight through in high and makes the CCKW situation even more ludicrous. The simpler straight through t'fer case from that truck will bolt right into the banjo diff CCKW t'fer case cross member. There is no doubt it will handle the power of the larger truck as it uses the same weight gears,shafts and bearings. In fact some of them interchange. This means that there was a perfectly useable, cheaper, simpler t'fer case in production at the same time without the 1.15:1 reduction.
I would really like for someone to explain why this was done without any guess work or half assed theories. I can come up with those on my own.
See how you do with this one Ranger.
Yours in friendly cage rattling.
David
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Gordon_M
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Re: Overdrive-Underdrive

Post by Gordon_M »

Well I'll throw two logs on this fire.

1. It may be that they were not at all interested in top speed ( or noise - tell me about it ), but in the range of ratios available to the driver.

2. It may also be that they just used whatever they had available, in production, and in sufficient quantity.


My favourite setup was my Chevrolet K43 which had the full original four speed transmission with straight-through on fourth and high range, but had a GMC 270 up front providing the power. That was unbeatable combination for my particular need at the time, but I will admit might have lacked ratios if it was fully loaded.
Gordon, in Scotland

( Now officially given up on any form of politics )
Forgotten Field
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Re: Overdrive-Underdrive

Post by Forgotten Field »

lacked ratios if it was fully loaded
Said it all there. The CCKW was set up for 10,000 lbs on the road, and it will take it on any grade. The CCKW will start out in 1st gear, low range, with almost any load you can put on it. There was a great video I saw of CCKW's being used to move houses when a highway was being moved. If you read the manual, the front wheel drive and low range are to be engaged when negotiating steep inclines in a loaded truck.

I haven't REALLY loaded my tipper yet. But rock clearing season is upon us soon, and I'm looking forward to seeing how it drives with a load. I have some real hills here on my property, with 30-40 foot elevation changes in 200-300 feet.
Nothing to see here, just move along...
motto
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My garage: Harley Davidson 42WLA, Dodge WC63, Dodge WC57, Chevrolet 1-1/2 ton 4x4. Originally an Earth Borer, Pole Setter. Drilling rig gone and truck now mounts a #7 Set Lifting Frame. i.e.: Light Wrecker.
Location: 70km from Melbourne Australia

Re: Overdrive-Underdrive

Post by motto »

I'm sorry fellows but I can't believe for one minute that it was a matter of suitable ratios. This is for a number of reasons. For highway cruising there would be no noticeable difference in speed with a straight through g'box/t'fer case set up. This has nothing to do with top speed. As stated, the o'drive is cancelled out by the underdrive when both units are in top.
The five speed Clark box has a miserable set of ratios anyhow with little steps from 1st to 2nd to3rd then a big jump to 4th and a little hop to 5th. To top it off they are weak and prone to problems in the 5th speed area after after they've done a bit of work.
Bottom end performance is an unlikely argument as I don't see that this is enhanced by the reduction in high range.
To once again compare the Chev 1-1/2 Ton driveline. Chev t'fer case low range 1.94:1 versus CCKW low range 2.63:1.
But that is not the whole story as the Chev g'box believe it or not has a lower 1st than the five speed Clark 7.23:1 as against 6.06:1.
If you are using the vehicle to its limits in low 1st you are attempting to do something the truck was never designed to do. Unless greatly overloaded they will always lose traction before they run out of grunt.
Keep in mind also that 350,000 Dodge 4x4s fought the entire war without even having the benifit of a low range in the t'fer case.
David
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Re: Overdrive-Underdrive

Post by RANGER »

An Army specification for a 2-1/2 Ton 6X6 Truck was submitted, the truck factory engineers went to work on it and the CCKW/US-6 resulted. The gear ratios were intentional. The country was tooling up for war, standardized trucks were designed with available parts or production capacity in mind, once the desired truck was developed, standardization came into being and the major components were purposely unchanged for the duration for the purpose of simplicity. The next generation of trucks, the M-Series of the 50's were designed to eliminate the shortcomings of the WWII trucks that served so well.
One ride in an M-35 powered by the Gold Comet or same in the M-54 5 Ton with R-6602 Engine , roll up windows, adjustable seats and power steering was proof that the next generation truck was a success in Tactical Truck Design, and just what the driver had been looking for. :wink:
The CCKW was not a highway cruiser, it had a mission to perform and high speed cruising was not the goal.
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Dozerman51
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Re: Overdrive-Underdrive

Post by Dozerman51 »

I might be wrong, but when the WWII trucks were produced;the Dodges, Chev's, GMC's etc, the speed limit on most roads in the US was 40-45 MPH anyway. This being said, they could keep up with traffic if need be. In convoy, I'm guessing the speed would have been in the 30-35 MPH range. Speaking of "Speed", I'm enjoying the heck out of driving my WC-12 on the freeway now. With the 4.30 ratios front and back, 900-16NDT's, I'm cruising at 55-60MPH's at around 2650RPM's. At least the other vehicles are not passing me up like I'm standing still. LOL. Final ratio comes out to 3.94 if you factor in the larger tires. "Sucking gas and hauling a..", but maybe, not to much longer with fuel prices going up like they are. :(
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Re: Overdrive-Underdrive

Post by RANGER »

The idea was to get the load there, speed was not a factor. The WWII 6X6 was not made for speed, the transfer cases and gearboxes were not made for high speed operation, they were designed for hard off road use.
US ARMY HONOR GRADUATE MECHANIC, Restorer of fine Jeeps, MV's, MVPA 40+yrs, DAV, Army Aircrew member, Donor to Military Museums & CAF, MV Hobby since 1945
Other Hobby- Army Air Force & Busting Big Ass Military Imposters-Good at it